Radars for automobiles have been introduced for driving-aid functions, geared rather more towards comfort, such as for example the adaptation to cruising speed for motorway use, termed the ACC (Adaptative Cruise Control) function, or “Stop and Go” in urban driving. They use millimetric waves, in particular the 76-81 GHz band.
By virtue of evolving technologies, current applications are also aimed at safety functions of anticollision type, and it is even envisaged that the fully autonomous vehicle will be with us in the relatively short term, perception of the environment being ensured by the association of a certain number of sensors, relying on various technologies: radar, video, infrared in particular.
On account of its all-weather capabilities, the radar remains within this framework a predominant sensor and its detection and discrimination capabilities must be extended to guarantee overall reliability of the system. As far as anticollision is concerned, the radar sensor must in particular be capable of distinguishing, among the fixed objects that it detects, those which correspond to roadway infrastructure elements, from those which correspond to stationary vehicles in the way which potentially constitute a risk of collision. In this context, it is in particular fundamental that it does not generate false alarms that might give rise to braking or to an emergency avoidance manoeuvre, without real cause, in particular when the vehicle is moving at high speed. This demands increased sensitivity and discrimination capabilities making it possible to appraise the situation a large distance ahead of the vehicle, typically greater than 200 m. It may also be necessary to detect the edges of roads.
In this context, the detection sensitivity and also the angular resolution and angular location capability in the horizontal plane and in the vertical plane must be optimized simultaneously, whilst the dimensions of the antenna are particularly constrained.
A technical problem to be solved is in particular to obtain sufficient sensitivity and angular discrimination capabilities, while preserving a simple antenna architecture and while limiting the volume of processing.
To date, this problem has not been solved. From a purely technical point of view, an ideal solution would consist, for a constrained antenna surface area, in disposing radiating elements or sub-arrays of radiating elements over the whole of the available surface area, these radiating elements or sub-arrays being fed in an individual manner by active transmit and receive modules. This solution would make it possible to optimize the radiation patterns with respect to angular resolution, while controlling the level of the sidelobes of these patterns in transmission and in reception simultaneously. Unfortunately, for technology and cost reasons, it is not accessible within the framework of a millimetric wave application intended for automobiles in particular.
This is why simpler and less efficacious solutions are currently implemented.
Radars use array antennas optionally comprising several transmit channels and several receive channels, and carry out by numerical computation the formation of several beams in reception. In this case, transmission is performed on one or more antennal arrays producing a relatively wide beam, typically of more than 20° in the horizontal plane and 10° in the vertical plane, and reception is performed simultaneously or sequentially on several sub-arrays covering this same angular domain.
This technique makes it possible to locate the various targets in the horizontal plane, or indeed in the horizontal plane and in the vertical plane, by comparing the signals received on the various beams.
In certain cases, different sub-arrays of antennas are switched over time in transmit mode and in receive mode so as to generate a diversity of radiation pattern with the objective of measuring the azimuth and the elevation of the targets or of increasing the angular resolution. Such a principle is described for example in FIG. 3 or FIG. 4 of the publication “Automotive Radar—Status and Trends” by Martin Schneider (Proceeding the German Microwave Conference GeM IC 2005). This switching is performed to the detriment of the efficiency of the waveform of the radar, which is then divided by the number of switchings. Moreover, the presence of switches in the microwave frequency chain gives rise to losses which degrade the sensitivity of the radar, and makes it necessary to increase the reception frequency band of the radar, this also being of such a nature as to degrade the sensitivity by increasing the noise power picked up by the receiver.
In other cases, transmission is performed by a transmit antennal sub-array and reception is performed simultaneously on several other receive antennal sub-arrays. This solution makes it necessary to distribute the reference oscillator at 76 GHz over the whole set of receive channels so as to perform the synchronous demodulation of the received signals, this being conceivable only on a small number of channels, having regard to the technological difficulties. Such a principle is described for example in FIG. 5 of the aforementioned publication “Automotive Radar—Status and Trends” by Martin Schneider. The complexity is further increased when beamforming must be performed in azimuth and in elevation.
In certain cases, the same sub-arrays are used simultaneously in transmission and in reception, such as for example described in the article “Millimeter-wave Radar Sensor Based on a Tranceiver Array for Automotive Applications” by Matthias Steinhauer et al., IEEE Transactions on Microwaves Theory and Techniques (vol. 56, Issue 2, February 2008). In such a solution the transmit receive coupling is very significant, and the sensitivity of the radar is strongly degraded by the leakage of the noise carried by the transmission in the various receivers. Moreover, if several transmit sub-arrays are activated simultaneously, it is necessary to transmit orthogonal waves on these various sub-arrays, this being rapidly complex if the number of channels is significant. In the same manner in reception, in order for the processing to remain simple and robust, it is necessary that, for a given sub-array, only the signal transmitted by this same sub-array be processed. The overall range budget is thus strongly degraded.